Inertial spin welding of thermoplastic and thermoplastic coated container parts

ABSTRACT

A thermoplastic container bottom is inertially spin welded into the interior of a cylindrical container body of paperboard internally lined with a layer of thermoplastic material. A body supporting mandrel has an expansible body engaging portion which is inserted into the body and subsequently expanded to define a cylindrical surface of a diameter exactly equal to that of the specified internal diameter of the body. A seating member at the end of the body supporting mandrel establishes the depth to which the container bottom is inserted into the body and is provided with a low-friction projecting resilient member which contacts the rotating bottom.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of an original applicationfield Feb. 13, 1981, Ser. No. 234,344, by Vincent E. Fortuna and DonaldN. MacLaughlin, entitled "INTERTIAL SPIN WELDING OF THERMOPLASTIC ANDTHERMOPLASTIC COATED CONTAINER PARTS", now abandoned.

BACKGROUND OF THE INVENTION

U.S. Pat. No. Re. 29,448 discloses methods and apparatus for inertialspin welding of thermoplastic container parts. As disclosed in thatpatent, two axially mating thermoplastic container parts arerespectively mounted upon axially aligned mandrels. One of the mandrelsis temporarily coupled to a rotary drive means to bring that mandrel andthe container part carried by the mandrel up to a predetermined rotativespeed, at which time the rotary drive is disengaged, the rotary inertiadeveloped maintaining the mandrel in rotation after the drive isdisengaged. The two mandrels are then moved toward each other and thetwo container parts carried by the respective mandrels seat with eachother. The friction developed by the relatively rotating container partheats the plastic material as it simultaneously brakes the relativerotation to melt the material to fuse the parts to each other when therelative rotation ceases and the parts are permitted to cool.

In U.S. Pat. No. Re. 29,448, the two parts being welded were both of athermoplastic material. This fact is worthy of note in that in order togenerate the frictional heat required to melt the plastic material, themating parts of the container must fit with each other with aninterference fit. Where both parts are formed from the samethermoplastic material, the achievement of an interference fit of thistype is not especially difficult in that the part dimension is quiteaccurately established in the forming machine and any subsequentdimensional changes due to thermal expansion or contraction where theparts are stored for any substantial period of time prior to assemblynormally affects both of the parts to substantially the same degree.

In recent years, there has been substantial usage of containers in whichthe container body or side wall is formed primarily of paperboard orcardboard, usually sealed at the opposite ends by metal tops andbottoms. Cans for motor oil and frozen orange juice are typical examplesof containers of this type. Where a paperboard container body isemployed, it is necessary to coat or line the interior of the paperboardbody with some liquid tight material, thermoplastic materials beingfrequently used for this purpose.

Where the paperboard container body is lined with thermoplasticmaterial, it has been proposed to employ a thermoplastic material forthe container bottom which has led to the discovery that such bottomscould be spin welded to the container body inasmuch as the containerbody has a layer of thermoplastic material on its interior surface.However, difficulties have been encountered in forming and maintainingthe thermoplastic coated paperboard bodies within dimensional tolerancesacceptable for such a spin welding operation. In order to apply thethermoplastic liner to the paperboard, the thermoplastic is normallyheated in order to bond it to the paperboard and subsequent coolingtends to shrink the material so that the container becomes undersized.Non-uniform shrinkage in storage also tends to occur, and the paperboardcontainers may, during preassembly handling and conveying operations,become slightly out of round.

The present invention is especially directed to methods and apparatusenabling the spin welding of thermoplastic bottoms to thermoplasticlined paperboard containers.

SUMMARY OF THE INVENTION

In accordance with the present invention, a mandrel for supporting athermoplastic lined cylindrical container body during a spin weldingoperation is provided with an expansible container body engagingassembly which, in a contracted position, can be axially inserted intothe container body. Body engaging members in the form of circumferentialsegments of a cylindrical surface are normally maintained in acontracted position, as by a circumferential garter spring. Inclinedinternal cam surfaces on each of the segmental members are slidablyengaged by a complementarily inclined actuating cam which, upon axialmovement relative to the segmental members radially expands themoutwardly to an expanded position. In this expanded position, the outercylindrical surfaces of the segmental members are accuratley located todefine a cylindrical surface of a diameter precisely equal to thespecified internal diameter of the body for spin welding purposes.

A container bottom of thermoplastic material is formed with an axiallyupwardly projecting peripheral flange whose major outer surface isdimensioned for the desired interference fit with the specified internaldiameter of the can body. The outer upper edge of this flange is taperedupwardly and inwardly so that the flange can be pressed up into theinterior of the container body with the desired press fit.

A seating member is mounted at the lower end of the body carryingmandrel to establish the depth to which the container bottom is insertedinto the can body. The seating member is provided with a low-frictionresilient member which normally projects axially downwardly below thebottom of the seating member. In the assembly operation, the bottom isfirst elevated by its mandrel into contact with this resilient member,and the bottom carrying mandrel is then engaged with the rotary drive tobring the bottom mandrel up to rotary speed.

Other objects and features of the invention will become apparent byreference to the following specification and to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view, with certain parts omitted or brokenaway, of an inertial spin welding machine embodying the presentinvention;

FIG. 2 is a detail cross-sectional view of an upper mandrel of themachine in FIG. 1;

FIG. 3 is a detail cross-sectional view taken on the line 3--3 of FIG.1;

FIG. 4 is a detail cross-sectional view taken on the line 4--4 of FIG.2;

FIG. 5 is a detail cross-sectional view taken on the line 5--5 of FIG.2;

FIG. 6 is a detail cross-sectional view taken on the line 6--6 of FIG.2;

FIG. 7 is a cross-sectional view of a container body and containerbottom to be spin welded by the machine of FIG. 1;

FIG. 8 is an enlarged detail cross-sectional view of the containerbottom and body indicating relative dimensions;

FIG. 9 is a detail cross-sectional view showing the container bottom andbody welded to each other; and

FIGS. 10-1 through 10-7 are schematic diagrams showing sequential stepsin the spin welding operation performed by the machine.

The machine of the present invention, taken as a whole, utilizes many ofthe features disclosed in U.S. Pat. No. Re. 29,448, the disclosure ofwhich is hereby incorporated by reference. In the following description,and in the drawings, many parts of the present machine are illustratedin a simplified form and described only generally inasmuch as similarparts and structures are shown and described in detail in U.S. Pat. No.Re. 29,448, to which reference may be had if further detail is desired.

Referring first to FIG. 1, the stationary frame of the present machineincludes a base designated generally 10 which fixedly supports avertically extending central post 12. A cylindrical cam track 14 isfixedly mounted as by a plate 16 upon the upper end of post 12 incoaxial relationship with the vertical axis of the post. A secondcylindrical cam track 18 is fixedly mounted on base 10. All of thestructure described thus far constitutes the stationary frame of themachine, all of the remaining shown in FIG. 1 is mounted for rotationabout the axis of vertical post 12, only a portion of the rotatablestructure being shown.

The rotary structure includes a central sleeve 20 mounted upon post 12for rotation relative to the post as by a series of bearings 22, sleeve20 carrying upper and lower tables 24, 26 fixedly attached to thesleeve.

In FIG. 1 there is shown only a single set of mandrels which include anupper mandrel assembly designated generally 28, supported generally fromupper table 24, and a lower mandrel assembly designated generally 30carried generally by lower table 26. In the actual machine, there areseveral sets of upper and lower mandrels disposed symmetrically aboutthe axis of the machine, this arrangement being shown in U.S. Pat. No.Re. 29,448 previously referred to.

A drive gear 32 is fixedly secured to the lower end of sleeve 20 todrive the sleeve and the various parts mounted thereon in rotation aboutpost 12. As the sleeve and upper and lower tables 34 and 26 are rotated,the upper and lower mandrels 28 and 30 move in a circular path aroundcentral post 12. During this movement, a roller 34 on lower mandrelassembly 30 moves along a groove 36 in lower cam track 18 to raise andlower, lower mandrel assembly 30 at appropriate points in its cyclicmovement around post 12. A similar roller 38 engaged in a similar groovein upper cam track 14 likewise vertically raises and lowers uppermandrel assembly 28, while a second roller 40 engaged with upper camtrack 14 operates an actuating device to be described below. Thevertical reciprocation of the mandrel assemblies is quite similar tothat described in detail in U.S. Pat. No. Re. 29,448, for whichreference may be had for further details.

Further details of upper mandrel assembly 28 are shown in FIGS. 2-5.

Referring first to FIG. 2, upper mandrel assembly 28 includes a rigidhollow sleeve 42 which is fixedly secured at its upper end to uppertable 24. A housing designated generally 44 is mounted on the exteriorof sleeve 42, as by slide bearings 46 for vertical reciprocatorymovement on sleeve 42. Housing 44 is suspended by a pair of support rods48, see FIG. 3, which are fixedly secured at their lower ends to flange50 of housing 44 and pass upwardly through openings 52 in upper table24. A cross member 54 is fixedly secured to the upper ends of supportrods 48 and serves as a mounting for cam roller 38 which, as best seenin FIG. 1, rides in the upper groove of cam track 14. Housing 44 is thussuspended by support rods 48 from cam roller 38 and the housing movesupwardly and downwardly in accordance with the path followed by roller38.

Returning now to FIG. 2, a vertical guide rod 56 is fixedly secured atits lower end to a second flange 58 on housing 44 and projects upwardlythrough an opening 60 in upper table 24 to vertically guide housing 44in movement.

Four container body engaging members 62 are pivotally suspended from thelower side of flange 58. As best seen in FIGS. 4-6, the four engagingmembers 62 have the conformation of a circumferential segment of ahollow cylinder, the circumferential extent of each body engagingsegment 62 being slightly less than one-quarter of the overallcircumference so that spaces such as 64 between the axially extendingedges of adjacent members 62 enable the members to be moved radiallyinwardly relative to one another to an outer diameter less than that oftheir outer surfaces. The outer diameter of the members 62 as viewed inFIGS. 4, 5 and 6 is equal (with a slight negative tolerance) to thespecified internal diameter of a cylindrical container body to bereceived upon the members 62. This particular diameter is of substantialimportance to the rpesent invention and will be discussed in furtherdetail below.

Referring now particularly to FIGS. 2 and 3, it is seen that four radialslots 66 are cut into the underside of housing flange 58, each slot, asbest seen in FIG. 4, overlying one of the body engaging members 62. Aprojection 68 at the upper end of each member 62 passes upwardly throughthe slot 66 and a horizontally extending pivot pin 70 pivotally suspendseach member 62 from housing 44 at the underside of flange 58. Pivot pins70 all line in a common horizontal plane and, as best seen from FIG. 4,are so oriented that they are all tangent to a common circle centered onthe central vertical axis of sleeve 42. A garter spring 72 is mountedwithin circumferential grooves 74 on the outer sides of each of themembers 62 and resiliently biases the lower ends of the four members 62radially inwardly toward each other so that the outer surfaces ofmembers 62, as viewed in FIG. 2, are normally inclined downwardly andinwardly toward each other.

Referring now particularly to FIG. 2, a downwardly and inwardly inclinedcam surface 76 is formed on the interior side of each member 62 near thelower end of the member. A cam member 78 having cam surfaces 80 ofcomplementary inclination is located between the member 62 and isfixedly mounted to the lower end of an actuating rod 82 which passesfreely centrally upwardly through sleeve 42 and is secured at its upperend (FIG. 3) to a bracket 84 which mounts cam roller 40. As best seen inFIG. 1, bracket 84 also slidably receives the upper end of guide rod 56.

Referring now particularly to FIG. 5, it is seen that cam member 78 isof a cross-shaped configuration when viewed from above, with camsurfaces 80 being located at the ends of each of the four arms of thecross-shaped configuration. The lower portion of housing 44 is providedwith four radial slots 86 through which the arm portions of cam member78 freely project.

Referring now particularly to FIGS. 2 and 6, seating member 88 isfixedly mounted to the lower end housing 44 by means such as welding oras by screws 90 (FIG. 6). A recess 96 is formed on the lower side ofseating member 88 and a resilient member 98, whose purpose will bedescribed in greater detail below is fixedly mounted in and projectsdownwardly from the recess. Member 98 can be compressed to lie entirelywithin recess 96. In order to reduce friction, member 98 may be madefrom or coated with a low-friction material such aspolytetrafluoroethylene (Teflon).

An ejector plate 100 having a circular opening 102 is mounted at thelower end of a plurality of support rods such as 104, support rods 104being fixedly mounted in table 24.

Referring now to FIGS. 7-9, in those figures are shown cross-sectionalviews of a container body and bottom which are to be spin welded by theapparatus described above. Referring first to FIG. 7, a container bodyCB of cylindrical shape is shown in cross-section. The container body CBis formed by a cylindrical tube of paperboard 106 whose interior surfaceis lined with a moisture barrier liner or coat of a suitablethermoplastic material 108 such as polyethylene, polystyrene, orpolypropylene. A container bottom B is formed of the same thermoplasticsynthetic plastic material with a bottom 110 and an integral upwardlyprojecting peripheral flange 112. As best seen in the enlarged view ofFIG. 8, flange 112, is formed with a downwardly and outwardly inclinedsurface at its upper end, because in order to perform the desired spinwelding operation, there must be an interference fit between the innersurface of the thermoplastic layer 108 on container body CB and theouter surface 116 of flange 112. The inclined surface 114 enables theflange to be pushed axially inwardly into the container interior with ashoehorn-like action.

A second upwardly projecting flange 118 preferably is located upon thecontainer bottom B to protect the lower edge of the paperboard portionof the container body, as best illustrated in FIG. 9, which shows thebottom and container body at the conclusion of the spin weldingoperation.

The container bottom B preferably is provided with a downwardlyprojecting flange 120 and a plurality of radially extending ribs 122extend from the inner side of flange 120 to the bottom surface of bottomB to provide a rotary drive coupling between the bottom B and lugsformed on the upper surface of lower mandrel 30.

THE OPERATION

A general sequence of operation of the apparatus described above isillustrated schematically in FIGS. 10-1 through 10-7.

The initial step in the operation is shown in FIG. 10-1 in which uppermandrel 28 is raised to its maximum elevation and cam actuating rod 82is likewise raised to its maximum elevation to position cam member 78relative to body engaging members 62 in the position shown in FIG. 2. Aspreviously described, vertical positioning of upper mandrel 28 and camactuator 82 is controlled by rollers 38 and 40 respectively, theserollers riding in grooves in the stationary upper cam track 14 (FIG. 1)to raise and lower the respective rollers as the upper and lowermandrels are rotated about the axis of central post 12 of the machineframe. The broken lines 38C and 40C in FIGS. 10-1 through 10-7approximately indicate the configuration of the cam tracks upon whichrollers 38 and 40 ride. Similarly, the broken line 34C indicates the camtrack upon which the lower mandrel positioning roller 34 moves.

With cam rod 82 and cam 78 in their maximum elevated position relativeto body engaging segments 62, garter spring 72 has radially contractedthe lower ends of segments 62 so that the cylindrical outer surfaces ofsegments 62 in FIG. 10-1 approximate a downwardly convergentfrustoconical surface having an outside diameter at its lower endsubstantially less than the internal diameter of a container body CB.

In FIG. 10-1, a container body CB is located on a feed table FT-1 inunderlying coaxial alignment with upper mandrel 28. A container bottom Bhas been fed from a second feed table FT-2 onto the upper end of lowermandrel 30.

In FIG. 10-2, upper mandrel 28 has been lowered to insert the containerbody engaging segments 62, fully into the interior of the container bodyCB. During this lowering movement of upper mandrel 28, cam actuating rod82 is simultaneously lowered so that no relative vertical movementbetween actuating rod 82 and upper mandrel 28 occurs, and the bodyengaging segments 62, located in the interior of container body CB inFIG. 10-2 are thus still in the radially contracted position as in FIG.10-1.

In FIG. 10-3, actuating rod 82 has been moved downwardly relative toupper mandrel 28. Referring briefly to FIG. 2, the lowering action ofrod 82 illustrated in the transition from FIG. 10-2 to 10-3 causes cammember 78 to move downwardly from the position shown in FIG. 2, thisdownwardly movement of cam member 78 causing its inclined cam surfaced80 to slide downwardly along the inclined surfaces 76 on body engagingmembers 62 to pivot the members 62 radially outwardly about theirrespective pivots 70 into full contact with the interior wall ofcontainer body CB.

As previously described above, the characteristics of container body CBand the process by which the container body is manufactured are suchthat at the time the container body is fed into the spin weldingapparatus, the internal diameter of the body is normally somewhatsmaller than its specified diameter. The container bottom B, on theother hand, is quite accurately formed to its specified diameter andmaintains this diameter quite accurately after its formation. In orderthat the spin welding of the container body to its bottom may beefficiently performed, it is essential that the container body be at itsspecified internal diameter at the time the container bottom is insertedinto the body so that the desired interference fit between these twoparts, one of which is rotating relative to the other, will generate thedesired amount of frictional heat to fuse the opposed thermoplasticsurfaces to each other.

As described above, the outer surfaces of the body engaging members 62of upper mandrel 28 are accurately machined to a diameter corresponding(with a slight negative tolerance) to the specified internal diameter ofcontainer body CB. The radial expansion of body engaging members 62, byactuation of cam rod 82 after the members have been inserted into thecontainer body, is likewise accurately regulated so that at theconclusion of the cam actuated expansion of body engaging members 62,the outer surfaces of these members are conformed to a cylindricalsurface of a diameter equal to the specified internal diameter of thecontainer body (with a slight negative tolerance). Thus, in FIG. 10-3 inconclusion of the expansion of body engagning members 62 by lowering ofcam actuator 82, the interior of the container body has been accuratelyexpanded to its specified diameter. The lower ends of body engagingmembers 62 are spaced upwardly slightly from the bottom of the containerbody to afford sufficient axial clearance for the insertion of flange112 of the container bottom C.

In FIG. 10-4, lower mandrel 30 has been elevated to position thecontainer bottom B closely beneath, but out of contact with, the lowerend of the container body CB supported upon the upper mandrel. In thisstep of the process, the lower mandrel 30 is engaged with a drive beltDB which drives the lower mandrel in rotation about its axis. The drivebelt DB operates along a portion of the circular path followed by thelower mandrel, see U.S. Pat. No. Re. 29,448 for details of this drivingarrangement. The container bottom B is rotatively locked to lowermandrel 28 by the webs 122 and thus rotates with the lower mandrel.

The next step in the sequence of operation finds upper mandrel 28 beinglowered slightly to seat the container bottom B within the bottom of thecontainer body. Just prior to this lowering step, lower mandrel 30passes out of engagement with drive belt DB, however, the rotary inertiaof lower mandrel 30 maintains the lower mandrel in rotation as thebottom inserting step is performed. Because the container body CBcarried by the upper mandrel 30 is held against rotation, the relativerotation between the engaged thermoplastic lined interior of containerbody CB and the rotating thermoplastic bottom B carried by the lowermandrel generates frictional heat melting the two engaged surfaces andat the same time exerts a braking action upon the rotating lower mandreland container bottom. This braking action swiftly brings the freelyrotating lower mandrel to a halt, the energy expended in the brakingoperation being converted to the heat which melts and, upon halting,fuses the container bottom to the container body.

In FIG.10-6, the lower mandrel has been lowered to its originalposition, the now fused container body and bottom are carried above anoutfeed table OF and cam actuating rod 82 has been elevated to permitthe body engaging members 62 to be restored to their contracted positionby garter spring 72.

In FIG. 10-7, upper mandrel 28 has been elevated to its originalposition to drop the container body CB onto the outfeed table, thisseparation of the container body from the upper mandrel having beenassisted by stripper plate 100 (FIG. 2).

While one embodiment of the invention has been described in detail, itwill be apparent to those skilled in the art that the invention may bemodified. Therefore, the foregoing description is to be consideredexemplary rather than limiting, and the true scope of the invention isthat defined in the following claims.

What is claimed is:
 1. Spin welding apparatus comprising upper and lowermandrel means mounted for vertical movement toward and away from eachother along a common vertical axis, said upper mandrel means beingadapted to releasably support an open ended cylindrical container bodyand said lower mandrel means being adapted to releasably support acontainer bottom of thermoplastic material having an axially upwardlyprojecting peripheral flange insertable into said container body with apress fit, first drive means for driving at least one of said mandrelmeans in rotation about said vertical axis relative to the other mandrelmeans, and second drive means for driving said mandrel means in movementalong said vertical axis to seat said container bottom within saidcontainer body while said mandrel means are rotating relative to eachother to insert and frictionally weld said container bottom in saidcontainer body; with radially expansible and contractable body engagingmeans mounted on the lower end of said upper mandrel means insymmetrically disposed relationship about said vertical axis, saidengaging means being normally disposed in a contracted position whereinsaid engaging means may be axially inserted into a container body,expanding means for radially expanding said engaging means to anexpanded position wherein the radially outer surfaces of said expandingmeans lie on a common cylindrical surface having a diameter equal to thespecified internal diameter of said container body and furthercomprising a seating member mounted at the lower end of said uppermandrel means and engagable with said container bottom to establish thedepth to which said container bottom is inserted within said containerbody by operation of said second drive means.
 2. The invention definedin claim 1 wherein said body engaging means comprises a plurality oflike body engaging members, each of said engaging members having anouter surface conformed to a circumferential segment of said commoncylindrical surface.
 3. The invention defined in claim 2 wherein saidengaging members are circumferentially spaced from each other when saidengaging members are in said expanded position.
 4. The invention definedin claim 3 further comprising pivot means at the upper end of each ofsaid engaging members mounting said members for pivotal movement on saidupper mandrel means about respective axes lying in a common horizontalplane and tangent to a circle centered on said vertical axis.
 5. Theinvention defined in claim 4 further comprising spring means adjacentthe lower ends of said engaging members resiliently biasing saidengaging members to their contracted position.
 6. The invention definedin any of claims 2, 3, 4, or 5 wherein said expanding means comprises anactuating cam mounted for vertical reciprocation relative to saidengaging members, said cam having a plurality of vertically inclined camsurfaces respectively slidably engaged with complementary inclinedsurfaces on each of said engaging members to force said engaging membersradially outwardly upon vertical movement of said cam in a firstdirection relative to said engaging members.
 7. Spin welding apparatuscomprising lower mandrel means for supporting and rotating athermoplastic container bottom having an upwardly projecting peripheralsurface, upper mandrel means for supporting a cylindrical containerbody, drive means for inducing rotation of said lower mandrel about avertical axis, and means for moving said mandrels relative to each otheralong said vertical axis to seat the peripheral surface of said bottomwithin the lower end of said body while said lower mandrel is rotatingto frictionally weld said bottom to said body; wherein said uppermandrel comprises a plurality of body engaging members symmetricallydisposed about said vertical axis, each of said body engaging membershaving an outer surface corresponding to an axially extending segment ofa cylindrical surface of a diameter equal to the specified internaldiameter of said cylindrical container body, mounting means mountingsaid body engaging means on said upper mandrel means for radialexpanding and contracting movement relative to said vertical axisbetween an expanded position wherein the outer surfaces of said bodyengaging members cooperatively define the major portion of a cylindricalsurface coaxial with said vertical axis and of a diameter equal to saidspecified internal diameter and a radially contracted position whereinsaid body engaging members may be axially inserted into the interior ofa container body, expanding means operable when body engaging membershave been axially inserted into a container body for expanding said bodyengaging members to their expanded position to mount said body on saidupper mandrel means and to size said body to said specified internaldiameter and further comprising a low-friction bottom seating membermounted on the lower end of said upper mandrel means for establishingthe depth at which said bottom is located within said container bodywhile being frictionally welded thereto.
 8. The invention in claim 7wherein said mounting means comprises pivot means at the upper end ofeach member mounting the member for pivotal movement relative to saidupper mandrel means about a horizontal axis, the horizontal axes of saidpivot means being tangent to a common circle centered on said verticalaxis, and spring means coupled to said members adjacent to the lowerends thereof resiliently biasing said members to said contractedposition.
 9. The invention defined in claim 8 wherein said expandingmeans comprises a cam member mounted in said upper mandrel means forvertical movement relative thereto, said cam member having a pluralityof cam surfaces inclined relative to said vertical axis, and actuatingsurfaces on the interior of the respective members adjacent the lowerends thereof slidably engaged with the respective cam surfaces to causeexpanding pivotal movement of said members upon vertical movement ofsaid cam member in a first direction.
 10. The invention defined in claim7 further comprising means defining a recess in the bottom of saidseating member, and a resilient low-friction pad member mounted in saidrecess and normally axially projecting below said seating member toengage said container bottom prior to the seating of said containerbottom against said seating member, said resilient pad member beingcompressible into said recess upon seating of said container bottomagainst said seating member.
 11. The invention defined in claim 10wherein said pad member further comprises an external surface ofpolytetrafluoroethylene.
 12. Spin welding apparatus comprising upper andlower mandrel means mounted for relative vertical movement along avertical axis, said upper mandrel means being adapted to releasablysupport a container upper part having a thermoplastic peripheral surfaceand said lower mandrel means being adapted to releasably support acontainer lower part having a thermoplastic peripheral surface adaptableto axially mate with said surface on said upper part with aninterference fit, first drive means for driving said mandrel means inrelative movement along said vertical axis to telescope said upper andlower part surfaces in mating relation, and second drive means fordriving at least one of said mandrel means in rotation about saidvertical axis relative to the other mandrel means to frictionally weldsaid surfaces, radially expansible and contractable part engaging meansmounted on an end of one of said mandrel means in symmetrically disposedrelationship about said vertical axis, said engaging means beingdisposable in a contracted position wherein said engaging means may beaxially inserted into one of said parts, and expanding means forradially expanding said engaging means to an expanded position whereinthe radially outer surfaces of said expanding means lie on a commoncylindrical surface having a diameter equal to the specified internaldiameter of said container part into which it is inserted.